1. Field of the Invention
This invention relates a high input impedance circuit used for the purpose of realizing, by a semiconductor integrated circuit, an amplifier having, e.g., an input impedance of several hundreds K ohms.about.several tens M(mega) ohms, which is necessary as a detector for taking out a very small voltage produced from a high impedance element.
2. Description of the Background Art
Generally, sensors used for various measurements tend to need considerably high input impedance. For example, it is now common to provide a shock sensing device to maintain reliability in writing/reading a hard disk drive, by detecting a vibration or a shock such that any error takes place during read/write operation and carrying out read/write operation for second time. The shock sensor, for example a ceramic element of a piezoelectric bimorph type, used to detect the shock has a high impedance, e.g. tens of picofarad(pF) and the shock sensing device must have an input buffer amplifier which can amplify a very small voltage produced from the high impedance element.
Hitherto, in order to realize high input impedance, a bias was applied, by, e.g., a resistor of 20M ohms to input of an amplifier (operational amplifier) to realize such a high input impedance. However, even if, e.g., an input current of an operational amplifier is 50 n(nano)A, a voltage drop of 1 volt takes place since the bias resistance value is so high. For this reason, as shown in FIG. 5, At was necessary to use an operational amplifier using an FET in a first stage where there is hardly an input current.
However, in order to realize such a function as an integrated circuit, since MOS structure is added, process of Bipolar-CMOS (BiCMOS) must be implemented to a semiconductor substrate for the purpose of manufacturing. In addition, in order to form a resistor of 20M ohms on a semiconductor substrate, chip area must be enlarged. Alternatively, since manufacturing is difficult from an economical point of view, it was necessary to externally mount such a resistor as a bias resistor.
FIG. 5 shows a conventional high input impedance circuit of an operational amplifier having MOSFET. An operational amplifier 10 has a positive input terminal and a negative input terminal, and an output terminal connected to an output terminal OUT. The positive input terminal is connected to an input terminal IN and is connected to one end of a resistor R1 of 20M ohms. Further, the positive input terminal is grounded through the resistor R1 and a power supply VB, and is biased by the resistor R1 and the power supply VB. In addition, an output signal is fed back to the negative input terminal. As stated above, in this conventional high input impedance circuit, input impedance of the operational amplifier is defined by the resistor R1.
Moreover, FIG. 6 shows another conventional example of an impedance circuit described in the Japanese Patent Application Laid-open Publication No. 61-18019 (1986) (Japanese Patent Publication No. 5-800005(1993). This circuit is a high input impedance circuit such that, e.g., base of a lateral PNP transistor Q20 is connected to base of an NPN transistor Q10 to deliver a bias voltage to emitter of the lateral PNP transistor Q20. Collector of the NPN transistor Q10 is connected to power supply (not shown), and its emitter is grounded through a current source 11. The base of the transistor Q1O is connected to an input terminal 12, and is connected to the base of the lateral transistor Q20. Collector of the transistor Q20 is grounded and emitter thereof is grounded through a resistor R and a power supply V1. In order to allow operating voltage (base voltage) of the transistor Q10 to be stable, when current amplification factor (hfe) of the transistor Q20 is caused to be smaller that of the transistor Q10, since it is possible to constitute a circuit having input impedance of 50K ohms or more only by, e.g., resistor R of about 1K ohms and a single lateral transistor, increase in chip area can be suppressed. However, in order to obtain, e.g., high input impedance of 20M ohms .about.60M ohms, a resistor above 1M ohms is required. Therefore, employment of such a resistor is difficult in view of allowing such a circuit to be integrated within a semiconductor substrate.
As previously described, in order to allow an amplifier of which input impedance is high impedance of several hundreds K ohms.about. several tens M ohms to be within a semiconductor substrate, high-resistance (high impedance resistor) of the order of M ohms or more is required, and such a resistor had to be externally attached (mounted) separately from the semiconductor substrate on which the amplifier is formed. Moreover, even if attempt is made to implement integration, it was necessary to utilize BiCMOS process in which process steps would become complicated.